The invention relates to an apparatus for supplying supplementary air to the air induction tube of an internal combustion engine. The induction tube includes an air flow measuring device associated with a fuel-air mixture generator as well as an arbitrarily settable throttle valve and a bypass air line of variable cross section surrounding both of the elements in the induction tube.
In a known method for supplying a controlled amount of supplementary air in the above-described apparatus, the oxygen content of the exhaust gases is monitored and supplementary air is metered out through a bypass valve in dependence on the measured oxygen content of the exhaust gas. An electronic controller is used to receive signals related to the position of the throttle valve and the prevailing engine speed (rpm) so as to obtain a basic setting of the bypass valve which is then additionally adjusted in dependence on the signal from a device which measures the oxygen content of the exhaust gases. The controlled supply of air to a fuel-air mixture generated by a mixture generator may be admitted in principle through a bypass line which is parallel to the mixture generator or to the air flow rate measuring element but may also be admitted to a bypass line which is in parallel with the mixture generator and the throttle valve. The mixture generator is usually a carburetor having an air flow measuring member and a fuel dosing mechanism which meters fuel out in dependence on the measured air flow rate. In the first mentioned instance, i.e., when the bypass line goes around the mixture generator alone, the desired effect may not be achieved at engine idling speeds because there is practically no pressure difference available for supplying the additional air. The same is true for the second case whenever the engine is operated at full load and low rpm.